An ink-jet printer includes at least one print cartridge that contains ink within a reservoir. The reservoir is connected to a print head that is mounted to the body of the cartridge. The print head is controlled for ejecting minute droplets of ink from the print head to a sheet of print medium, such as paper, that is advanced through the printer.
Many ink-jet printers include a carriage for holding the print cartridge. The carriage is scanned across the width of the paper, and the ejection of the droplets onto the paper is controlled to form a swath of an image with each scan. Between carriage scans, the paper is advanced so that the next swath of the image may be printed. Sometimes, more than one swath is printed before the paper is advanced. In some printers, a stationary print head or array of print heads may be provided to extend across the entire width of the paper that moves through the printer.
The relative position of the print head(s) and paper must be precisely maintained to effect high-resolution, high-quality printing. This precision is especially important in the region known as the "print zone" of the printer, which is the space where the ink travels from the print head to the paper. Changes in the relative position of the print head and paper will cause the expelled ink droplets to land imprecisely on the paper and thus degrade the quality of the printed image.
One method of securing a sheet of paper for movement through a printer is to direct the paper against one side of a perforated belt. Vacuum pressure is applied to the other side of the belt and, thus, through the belt perforations to secure the paper to the belt. The belt, with secured paper, is moved relative to the print head and through the print zone where ink is printed on the paper.
The belt may be configured as an endless loop and secured between a pair of rollers that are mounted to the printer to drive the belt under tension. The upper surface of the belt transports the paper that is guided onto the belt. The porous belt moves over a support surface that includes vacuum ports through which the vacuum pressure is applied to the belt and to the paper that is carried by the belt.
The speed with which the print media is moved through a printer is an important design consideration, called "throughput." Throughput is usually measured in the number of sheets of imaged print media moved through the printer each minute. A high throughput is desirable. A printer designer, however, may not merely increase throughput without considering the affect of the increase on other print quality factors.
For instance, one important factor affecting the print quality of ink-jet or other liquid-ink printers is drying time. The print media movement must be controlled to ensure that the liquid ink dries properly once printed. If, for example, sheets of printed media are allowed to contact one another before ink is adequately dried, smearing can occur as a result of that contact. Thus, the throughput of a printer may be limited to avoid contact until the sheets are sufficiently dry. This potential for smearing is present irrespective of whether ink is applied by a scanning technique or by other methods, such as stationary print head arrangements that effectively cover an entire width of the print media.
In addition to throughput, an ink-jet printer designer must be concerned with the problem of cockle. Cockle is the term used to designate the uncontrolled, localized warping of absorbent print media (such as paper) that occurs as the liquid ink saturates the fibers of the paper, causing the fibers to swell. The uncontrolled warping causes the paper to move relative to the print head, changing both the distance and angle between the print head and the paper. These unpredictable variations in distance and angle reduce print quality.
Heat may be applied to the print media in order to speed the drying time of the ink. If heat is applied to the media, it is useful to have it applied so that the media is heated as it is moved through the print zone during a printing operation. The heat rapidly drives off (evaporates) a good portion of the liquid component of the ink so that cockle is unable to form, or at least is minimized.
An effective way to heat the print media is by conduction, in a manner that will not overheat the print head nor interfere with the trajectory of the droplets expelled from the print head. This can be accomplished by heating the underside of the belt by conduction, which heat is thus transferred to the media carried by the belt.
If the part of the belt in the vicinity of the print media is unevenly heated, undesirable ripples may be produced in the belt. More particularly, rippling or buckling in the belt happens when a heated portion of the belt expands against the adjacent, relatively cooler portion of the belt. If the temperature difference or gradient is large enough, the cooler portion constrains the expansion of the belt in the plane of the belt. As a result, thermal stress is introduced into the belt, and the belt responds by buckling away from the belt support surface.
The occurrence of such belt buckling or rippling in the print zone is undesirable because the portion of the print media that overlays the ripple will be lifted slightly by the ripple toward the print head. As noted, such uncontrolled changes in the distance between the media and the print head can reduce print quality. Moreover, conductive heat transfer is substantially reduced or lost in the region where the belt moves away from the heated support surface. The uneven heating of the media resulting from such heat loss leads to additional print defects.
The present invention is generally directed to techniques for managing the thermal stresses introduced into a heated belt of the type just described so that the portion of the belt that transports the print media through the print zone remains free of ripples.
In one approach to the invention, the temperature of the belt is controlled to ensure that the temperature gradient of the belt remains below a predetermined threshold in the vicinity of the print zone so that the induced thermal stresses remain below a level that would create buckling.
As another approach to the present invention, the belt is bent in the print zone to create a greater (i.e., stiffer) area moment in the belt for countering the thermal stress that would otherwise produce buckling of the belt.
Both apparatuses and methods for carrying out the invention are described. Other advantages and features of the present invention will become clear upon review of the following portions of this specification and the drawings.